1. Field of the Invention
The invention in general relates to the field of sonar signal processing, and more particularly to the time varying processing of the sonar signals.
2. Background Information
In a sonar system, acoustic pulses are transmitted through the water column toward a target area and acoustic returns from the target area are picked up by a receiver transducer and processed for display.
In one type of sonar system which can form one or more receiver beams, the receiver transducer is comprised of a plurality of individual elements each of which generates an output signal in response to the received acoustic returns. The output signals are provided to individual preamplifiers, corresponding output signals of which, are processed, either by analog circuitry or digital circuitry to form one or more beam signals for display.
Return signals may vary due to unknowns such as temperature, salinity and clarity of the water column. Further, if bottom returns are involved, such as in side looking sonar systems, different bottom types such as mud, sand or rock will return different signals. The preamplifiers and other signal processing circuitry therefore must have enough bandwidth to accommodate for the variations.
To accommodate for attenuation through the water column and for expected returns, a time varied gain circuit is generally provided to vary the gain of the preamplifiers as a function of time after an acoustic transmission. The varied gain function is predetermined and fixed. In addition, in order to decrease the dynamic range of a beam signal for display purposes, and to account for unknowns, there is generally provided an adaptive time varied gain circuit which modifies the beam signal based upon a history of the signal level present in previous returns. That is, gain function is not fixed but is adaptive to present conditions.
Although the beam signal dynamic range is decreased for display, the front end circuitry for amplifying and signal processing still requires a wide dynamic range due to potential error in the fixed time varied gain curve.
The present invention provides for an improved time varied gain correction which can accommodate for unknowns in the water column and various bottom types.